To perform a cut with a hydraulic shearing machine at least one shear blade has to be moved in the direction toward another shear blade with a force which is named overall actuating-force, that overall actuating-force being sufficient to overcome the resistance of the material to be cut, that resistance being named cutting force. When shearing wide metal plates and strips, a straight cut of the full width of material would require an enormous overall actuating-force which would not be practical. Consequently, shearing machines for shearing wide material generally use either a raked shear blade, or a curved shear blade for a rocking type shearing action.
A raked shear blade type shearing is illustrated in simplified cross section in FIG. 1. The upper raked shear blade 1 moves down relative to the lower straight shear blade 2 in order to cut the material 3. The overall actuating-force necessary is much reduced compared to a straight cut of the full width of the material because the width of the material which is being cut 4 at any particular point during the whole cutting cycle is much smaller than the full width of the material 3.
A rocking type shearing action is illustrated in simplified cross section in FIG. 2. The curved upper shear blade 5 rocks in such a way that it approximates a rolling type movement where the lowest point of the curved upper shear blade stays at approximately the same vertical height whilst moving horizontally across the material 6 which is being cut. The width of the material which is being cut 7 at any particular point during the whole cutting cycle is much smaller than the full width of the material which is being cut 6. During shearing, the lower straight shear blade 8 does not move.
GB 2405118 A describes a hydraulically actuated shearing machine which achieves a rocking type shearing action by using a curved shear blade and two separately controlled hydraulic cylinders. A simplified cross section is illustrated in FIG. 3. The piece of material that is to be cut 9 is positioned between an upper curved shear blade 10 and a lower straight shear blade 11. The upper curved shear blade 10 is attached to an upper support beam 12 and the lower straight shear blade 11 is attached to a lower support beam 13. Two hydraulic cylinders 14 and 15 of a hydraulic actuating-mechanism are connected between the upper support beam 12 and the lower support beam 13. Each of the hydraulic cylinders 14 and 15 engages the upper support beam 12 in one engaging-area, hydraulic cylinder 14 in the engaging area on the left end of upper support beam 12, and hydraulic cylinder 15 in the engaging area on the right end of upper support beam 12. By controlling the stroke of hydraulic cylinders 14 and 15 separately but in a synchronized manner the upper shear blade 10 can be made to execute a rocking type shearing action. The same type of shearing machine could be used with raked shear blades instead of curved shear blades in which case the two hydraulic cylinders move synchronized in the same direction to achieve the cutting action.
A consequence of using a raked shear blade or a rocking type shearing action is that the distance which the shear blade has to move to complete the cut is much greater than for a straight full width cut with a straight shear blade. For such a straight cut the shear blade only has to move through at most the full thickness of the material which is being cut. With a raked shear blade the distance the shear blade has to move is the thickness of the material plus the width of the material multiplied by the tangent of the rake angle. Typically the rake angle is only about 2 degrees and consequently for wide material the shear blade movement required is many times greater than for a straight cut. Similarly for a rocking type shearing action as illustrated in FIG. 3 the movement of the hydraulic cylinders 14 and 15 is many times greater than would be required for a straight full width cut.
The actuating-force of a hydraulic cylinder 14 or 15 which is operated with the cylinder rod in tension is the product of its annulus area multiplied by the available supply pressure of the hydraulic fluid. In a shearing machine according to FIG. 3 the combined annulus area of the hydraulic cylinders 14 and 15 must be sufficient to generate the required overall actuating-force for the thickest and strongest material that is to be cut with the available supply pressure of the hydraulic fluid.
In a practical shearing line for metal plates and strips it is important that the throughput of the shearing machine can keep pace with the other production units which deliver the material to be cut and process the cut material. Throughput is the mass of material processed per time unit, i.e. output per time unit. The throughput of a shearing machine depends on a number of factors including the width and thickness and strength of the material being cut, the number of cuts required, the time it takes to perform the complete cut, which is called the cutting cycle time, and the time it takes to reset the shear blades which are moved for shearing to their starting position and to move the piece of material between cut positions, which is called the reset-time.
The strength of a material is defined by parameters such as yield strength and elongation to fracture.
As a result of these factors a hydraulically operated shearing machine like the one in FIG. 3 has three primary parameters which determine the necessary size of the hydraulic fluid pump system and of the valves which supply fluid to the hydraulic cylinders. The first parameter is that the combined annulus area of the hydraulic cylinders must be sufficient to generate the overall actuating-force required for the strongest and thickest material that is to be cut. The second parameter is that the stroke of the hydraulic cylinders must be sufficient to cut the widest and thickest material. The third parameter is that for all kinds of materials to be cut the shearing machine must be able to provide cutting cycle times and reset-times which allow fulfilling the throughput demands. The sum of cutting cycle time and reset-time must permit the hydraulic shearing machine to perform the number of cuts per time unit necessary for the desired throughput. For example, to achieve sufficient throughput a shearing machine for metal plates typically must be able to work with a cutting cycle time of about 3 seconds and a reset-time of about 7 seconds.
For thin and/or narrow material per ton of output more cuts are required than for thick or wide material, and consequently the same throughput requires more cuts per time unit than for thick and/or wide material. Therefore, the hydraulic system of pumps, valves and cylinders of a shearing machine like the one in FIG. 3 has to be dimensioned such that it can provide the short cutting cycle time and reset-time required for the thinnest and/or narrowest material to be cut.
Hence for cutting materials with a wide range of width, thickness and strength with a certain throughput in a hydraulic shearing machine like the one in FIG. 3, the hydraulic cylinders must have a large annulus area and a large stroke and are required to move with the short cutting cycle time and reset-time which is needed for the thinnest or narrowest material. Hydraulic cylinders with large stroke and large annulus area have a large volume. Consequently, the volume of hydraulic fluid to be pumped for moving the shear blades is large and has to be pumped quickly, which requires large hydraulic pumps and valves. A hydraulic system with large pumps and valves is very expensive and needs a lot of space, and the operation of large pumps and valves is energy-intensive and high-maintenance.
FR2212771 describes a hydraulic shearing machine with a raked blade in which two hydraulic cylinders with common stem engage the support beam of a raked blade. Both hydraulic cylinders are hydraulically connected. When during cutting the pressure in the firstly used cylinder rises above a threshold value a valve in delivery line for hydraulic fluid to the cylinders opens and the second cylinder begins to operate in addition to the firstly used cylinder. If the pressure in the firstly used cylinder does not rise above the threshold value, that valve does not open, consequently the second cylinder is not supplied with hydraulic fluid under pressure and only the firstly used cylinder is in operation. For fast resetting the blade after cutting only one of the hydraulic cylinders is filled with hydraulic fluid under pressure. Hence, FR2212771 provides a method of operating a hydraulic shearing machine with a raked blade with variable actuating forces, and offers the possibility to quicken the resetting movement by using only one of several hydraulic cylinders present. However, for a hydraulic shearing machine with a curved blade the teaching of FR2212771 is not applicable, because to achieve a correct rocking type shearing action with a curved blade it is required that the movement of the two ends of the curved blade is controlled and synchronized precisely. If the system described in FR2212771 was used on each end of a curved blade, the movements could not be properly synchronized because for a given hydraulic flow the speed of the downward movement of the blade in FR2212771 depends on whether or not the pressure in the firstly used cylinder exceeds the threshold.